In operating a continuous casting facility, it is necessary to detect the molten metal surface level in the continuous casting mold and stably control the molten metal surface level. This is because the internal quality of a cast slab can be improved by preventing overflow of molten steel and roll-in of suspended substance. The molten metal surface level refers to the height position of the surface of the molten steel. As a technique of detecting the molten metal surface level in the continuous casting mold, there are techniques described in Patent Literatures 1, 2. Note that the continuous casting mold is abbreviated as a mold as needed in the following description.
In Patent Literature 1, the following technique is disclosed. A plurality of temperature measurement elements are embedded in the mold at regular intervals along the casting direction of the mold (the height direction of the mold). The time change rate of the temperature at a point of each temperature measurement element is calculated to detect a temperature measurement element (n) exhibiting the maximum value of the time change rate. A position exhibiting the maximum value of a quadric curve linking the time change rate of the temperature measurement element (n) and the time change rates of two temperature measurement elements (n−1), (n+1) adjacent to the temperature measurement element (n) is obtained, and the position is regarded as the molten metal surface level.
Besides, in Patent Literature 2, the following technique is disclosed. A plurality of thermocouples are embedded in the mold at intervals along the casting direction of the mold (the height direction of the mold). For detecting the molten metal surface level, giving an initial temperature distribution and deciding a temporary molten metal surface level (division position) are performed first. Upon decision of the temporary molten metal surface level, the maximum heat flux and the minimum heat flux at the temporary molten metal surface level are calculated by analysis of heat conduction inverse problems using the temperature changes measured by the thermocouples. The maximum heat flux and the minimum heat flux at the temporary molten metal surface level are calculated with the temporary molten metal surface level changed. Among the calculated positions of the temporary molten metal surface levels, the temporary molten metal surface level where the difference between the maximum heat flux and the minimum heat flux defined by performing experiments in advance is smallest is regarded as the actual molten metal surface level.